1. The Field of the Invention
This invention relates to rotating wing aircraft, and, more particularly to rotating wing aircraft relying on autorotation of a rotor to provide lift.
2. The Background Art
Rotating wing aircraft rely on a rotating wing to provide lift. In contrast, fixed wing aircraft rely on air flow over a fixed wing to provide lift. Fixed wing aircraft must therefore achieve a minimum ground velocity on takeoff before the lift on the wing is sufficient to overcome the weight of the plane. Fixed wing aircraft therefore generally require a long runway along which to accelerate to achieve this minimum velocity and takeoff.
In contrast, rotating wing aircraft can take off and land vertically or along short runways inasmuch as powered rotation of the rotating wing provides the needed lift. This makes rotating wing aircraft particularly useful for landing in urban locations or undeveloped areas without a proper runway.
The most common rotating wing aircraft in use today are helicopters. A helicopter typically includes a fuselage, housing an engine and passenger compartment, and a rotor, driven by the engine, to provide lift. Forced rotation of the rotor causes a reactive torque on the fuselage. Accordingly, conventional helicopters require either two counter rotating rotors or a tail rotor in order to counteract this reactive torque.
Another type of rotating wing aircraft is the autogyro. An autogyro aircraft derives lift from an unpowered, freely rotating rotor or plurality of rotary blades. The energy to rotate the rotor results from a windmill-like effect of air passing through the underside of the rotor. The forward movement of the aircraft comes in response to a thrusting engine such as a motor driven propeller mounted fore or aft.
During the developing years of aviation aircraft, autogyro aircraft were proposed to avoid the problem of aircraft stalling in flight and to reduce the need for runways. The relative airspeed of the rotating wing is independent of the forward airspeed of the autogyro, allowing slow ground speed for takeoff and landing, and safety in slow-speed flight. Engines may be tractor-mounted on the front of an autogyro or pusher-mounted on the rear of the autogyro.
Airflow passing the rotary wing, alternately called rotor blades, which are tilted upward toward the front of the autogyro, act somewhat like a windmill to provide the driving force to rotate the wing, i.e. autorotation of the rotor. The Bernoulli effect of the airflow moving over the rotor surface creates lift.
Various autogyro devices in the past have provided some means to begin rotation of the rotor prior to takeoff, thus further minimizing the takeoff distance down a runway. One type of autogyro is the “gyrodyne,” which includes a gyrodyne built by Fairey aviation in 1962 and the XV-1 convertiplane first flight tested in 1954. The gyrodyne includes a thrust source providing thrust in a flight direction and a large rotor for providing autorotating lift at cruising speeds. To provide initial rotation of the rotor, jet engines were secured to the tip of each blade of the rotor and powered during takeoff, landing, and hovering.
At high speeds, the direction and orientation of an autogyro may be readily controlled using conventional control surfaces such as ailerons, rudders, elevators, and the like, that are exposed to air flow over the airframe of the autogyro. Pitch and roll may also be controlled by cyclically altering the pitch of the blades in order to increase the lift at a certain point in the rotation of each blade. Pitch and roll may also be controlled by altering the angle of the mast coupling the rotor to the airframe.
In an emergency landing when an autogyro has lost power, the airspeed of the autogyro is likely to be low due to a lack of propulsion. Where cross winds are present yaw control may be critical in order to maintain the autogyro aligned with a runway. At low airspeeds, pitch and roll may still be accomplished using cyclic pitch and mast tilt controls inasmuch as the rotor typically is still auto-rotating.
However, yaw control is not readily accomplished at low air speeds using conventional control surfaces. Control surfaces, such as rudders, may not have sufficient airflow thereover at low speeds to induce a yaw moment. In addition, autogyros typically do not have a tail rotor coupled to the engine to counteract torque exerted by the engine on the rotor as do helicopters.
In view of the foregoing, it would be an advancement in the art to provide means for controlling yaw of an autogyro at low speeds and, in particular, for controlling yaw of an autogyro in the event of engine failure.